Process for applying a protective coating to a magnesium surface



wane -Q y 1960 w. H. KAPFER ET AL 2,935,423

PROCESS FOR APPLYING A PROTECTIVE COATING TO A MAGNESIUM SURFACE Filed Sept. 25, 1956 CONDITIONS OPERATIONS MATERIALS Mg surface is immersed (A) 2 7 Acetic Acid solution for IS seconds and then Cleansing water rinsed.

The Mg surface is rinsed (B) 50-95 parts by weight in a preheated I0OC bath Coating of Sodium Pyrophosphate.

for 5 minutes. l-5 parts by weight of The PH of the solution is Sodium Fluoride.

controlled so as not to 4-40 parts by weight of diminish appreciably. Nickel Sulfate.

30 seconds (C) Cold Water Rlnsing Air Drying The Mg surface is immersed (E) 5-4070 by weight of an in an aqueous solution Coating aqueous solution of for lOmlnutes at 70C. Iron Phosphate.

30 seconds (F) Cold water Rinsing INVENTOR. William H. Kant" Max Kronstein ML.MW

- ATTORNEY PROCESS FOR APPLYING A PROTECTIVE COAT- ING TO A MAGNESIUM SURFACE William H. Kapfer, Hastings on Hudson, and Max Kronstein, New York, N.Y., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 25, 1956, Serial No. 612,076 3 Claims. Cl. 117-71 The present invention relates to the art of coating magnesium and more particularly to novel and improved techniques for applying protective coatings to magnesium and the alloys thereof to render the metal surface passive to oxidation and corrosion;

The protection of magnesium from corrosion is a well known problem in the arts due to the tendency of magnesium surfaces to rapidly iform coatings ofmagnesium oxides and hydroxides on contact with moist air or on exposure to salt fog. The formation of these corrosion products often progresses within a short time to a complete loss of metal substance at localized areas, and it has; been found that deterioration of the metal surface cannot be prevented by a mere simple application of a suitable organic paint or the like.

In the past various chemical" retreatrnents of the magnesium surface have also been suggested and introduced into the art to retard corrosion. Typical pretreatments, known heretofore, consist of an extensive chemical surface cleaning and acid pickling operation followed in some cases by a subsequent surface sealing procedure based on the use of a suitable chromate or dichromate immersion or the like. These pretreating and sealing procedures which are preferably carried out at controlled temperature and pH conditions to produce a uniform film on the magnesium surface, provide a surface condition on the magnesium that takes on characteristics of a partly soluble gel which provides a more reliable base for the organic paint or coating than an untreated magnesium surface. Nevertheless, irregularities in coatings thus applied have been found to occur frequently thereby contributing to local and general premature corrosion failures.

It "is a principal object of the present invention to provide novel and improved techniques for the surface protection of magnesium and its alloys by interacting the magnesium surface with solutions of other metal ions and compounds to produce a'protective metallic coating on the original magnesium surface.

It is a further object of the present invention to provide upon a magnesium or a magnesium alloy surface a novel and improved inorganic coating or film which is a deriva- 2,935,422 Patented May 3, 1960 comes better understood by reference to the following dedetailed description:

In the drawing, the single figure illustrates diagrammatically the series of steps employed in the invention. Inthe practice of the present invention, it is ordinarily ice desirable first to properly prepare the metal surface prior to the application of the protective coating. In this way a more uniform reactive surface may be obtained for ultimate exposure to the protective immersion fluid. Thus, old corrosion areas, earlier protective coatings, grease, and other contaminations of the surface should be removed to avoid local differences in contact with the immersion fluids.

It is to be understood that it would be possible to use the immersion fluids themselves in some instances for the desired surface preconditioning of the metal surface. This 7 practice, however, would cause some degree of contamination of the treating fluid, decrease the uniformity of the ultimate deposit, and shorten the useful life period of the fluid as a coating solution. Thus, it is preferable to preclean the metal surface using another suitable precleaning process and to apply the new metal surface coating thereafter. I

Among the types of surface preparations which were found suitable-for the purpose of precleaning are the following: liquid or vapor solvent cleaners, alkali cleaners, and other'conventional industrial cleaners known in the artfor the removal of grease contaminations; a suitable pickling operation such as immersion for 15 seconds in a 2 percent aqueous acetic acid solution followed by a water rinse; or-immersion for seconds in a solution containing 110 grams of aluminum chloride dissolved in a liter of water followed by a water rinse; or the application of successive pickles as for instance the above acetic acid pickle anda subsequent aluminum chloride pickle, each followed by a water rinse. It has been found that the metal surfaces prepared in this manner are more uniform and better suited to the subsequent metallic deposit.

' A number of particular metal deposit solutions which are next applied in accordance with the present invention were found to provide improved protective coatings for the magnesium metal surface. Some such solutions which were found to provide successful deposits were based on solutions of cadmium salts, zinc salts, chromium salts,

nickle salts, aluminum salts, a combination of cadmium and chromium salts, and combinations of other metal saltsi Many of these salt solutions exhibited their own peculiar characteristics, which made them more suitable for cercadmium. and chromium salts, were found to provide bettive of another metal substance without the application It is a further object of the present invention to apply a'novel and improved metallic deposit or coating upon a'.

magnesium or magnesium alloy surface-directlyorjto apply" the same after a chemical cleaning or pickling or chemical treating operation to increase the surface. reactivity of the magnesium. 1 Other objects and many of the attendant advantagesof ter results than others in use without the subsequent chem-h cal surface modification or sealer operation which are disclosed more fully hereinafter. Deposits obtained from solutions based on zinc salts were found very sensitive to variations in the pH of the solution and generally less suitable as a paint base prior to a chemical modification or seal. f

The form of the metals preferably used in the preparation of the solution is any form which allows enough metal. ions to enter the solution; that is, any of the salts or compounds of the metal having sufficient water solubility in the final treating solution. The most desirable metal salts used Were'in the form of sulfates or nitrates,

' but others such as cyanides, fluorides, etc. were also sucthis-invention will be readily appreciated as the same becessfully used. H

-. The solutions were compounded using varying concentrations of the metal salts together with. varying; amounts of pyrophosphate' imdistill'ed water. Small amounts of Grams Grams Grams Grams Grams (a) Cadmium sulfate 18.5 27.4 27.4 36. 36.5 Sodium pyrophsphate.. 394. 0 200. 0 394. 0 200.0 394. 0 Sodium fluoride-.. 10.0 10.0 10.0 10.0 10. 0 (Distilled water to make 2,000 ml. in all cases.) (b) Copper sulfate 19. Sodium pyrophosphate 200. 0 Sodium flouride 10. 0 (Distilled water to make 2,000 ml. in all cases.) (4:) Cadmium cyanide 80. 0 Sodium pyrophosphata. 120.0 Sodium fluoride 40. 0 (Distilled water to make 2,000 mi. in all cases.)

Chromium sulfate 72.2 Sodium pyrophosphate 200.0 Sodium fluoride 10.0 (Distilled water to make 2,000 ml. in all cases.) (e) Ghromic nitrate 60. 0 Sodium pyrophosphate" 200.0 Sodium fluoride l0. 0 (Distilled water to make 2,000 mi. in all cases.) (I) Nickel sulfate (N is 04). 90. 0 70.0 Sodium pyrophosphata. 200. 0 200.0 Sodium fluo 10.0 10.0

(Water to make 2,000 ml.) (9) Aluminum nitrate 94. 0 125.0 10.0 10.0 196. 0 195.0 1,700.0 1,670.0 60. 0 60. 0 80.0 62.0 10.0 200.0 Sodium pyrophosphate 200.0 10.0 (Water to make 2,000 mi.)

Each of the above solutions has been used sucessfully in accordance with the present invention with and without the prior pickling or cleaning operation, described above, to provide a uniform effective protective metallic coating on the surface of magnesium and magnesium alloy metals. The process of the application of the metallic deposit from each of the above solutions was characterized by the fact that no external electrical energy is needed to provide the desired coating. The process is also characterized by the fact that no additional metal matter is needed to induce the inter-reaction between the magnesium or magnesium alloy surface and the fluid, although the presence of such metals was not found to destroy theutility of'the process.

As. a more specific working example of the application of a particularly successful protective metallic deposit on a group of magnesium panels, the panels were first precleaned and pickled for seconds in a 2 percent aqueous acetic acid solution and water rinsed. The panels were then immersed for five minutes in a preheated 100 C. bath containing 18.5 g. of cadmium sulfate, 394 g. of sodium pyrophosphate and 10 g. of sodium fluoride in 2000 ml. of distilled water. During the immersion operation the pH of the solution was controlled so as not to diminish appreciably. After the immersion treatment the panels were rinsed in water for seconds and air dried.

Some of the above panels were then successfully tested and used as finished products, others were used as a base for paint and lacquer applications, and still others were chemically modified or sealed in a manner more fully described hereinafter prior toultimate test as a finished product.

This new magnesium surface system was found to be superior in corrosion resistance to untreated magnesium surfaces. A further increase in the life of the new system, especially its paint bonding life, was observed when the new surface system was further modified chemically to increase its uniformity, reduce its degree of porosity, and decrease the amounts of soluble matter not desirable in a 21 the final state. Treatments of this kind which were used successfully included the following examples:

Example I um phosphate mixtures and acted as modifiers and accelerators of the same.

Example II Another form of surface modification was carried out upon the panels described in column 3 by surface pickling the same with a metal salt solution of 40 grams of ammonium vanadite in ml. of water. In this particular application the panels were immersed in the solution at room temperature and the temperature was then increased to 70 C. and held for 10 minutes. Immediately following this imersion, a 30 second cold water rinse using running water was applied.

Example III Another surface modification of the panels in column 3 was obtained by immersing the panels for 4 minutes at room temperature with a surface modifying solution of a phosphate or fluoride compound followed by a rinse for 30 seconds in cold water.

Example IV Another very successful method of modifying or sealing the new system consisted of immersing the panels described in column 3 in a dicnromate sealer of 570 grams of sodium dichromate and 8.6 grams of calcium fluoride dissolved in one gallon of water to which 1 ml. of a wetting agent, Tergitol 08 (a 40% aqueous solution of the sodium sulfate derivative of 2-ethy1 hexanol) is added. The panels were immersed at approximately 98 C. for about 30 minutes, rinsed in cold water for 30 seconds, and allowed to dry.

The magnesium surfaces prepared by the above described applications of new metal deposits with or without the subsequent application of the surface modification have been found to differ from magnesium surfaces used in the past in that the manner in which the processes can be adapted to obtain the desirable high degree of uniformity and water resistance is less critical. Similarly, the new coating systems have also been found to exhibit a longer useful life even whenexposed to a corrosive environment such as salt spray fog or water.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A process for applying a protective coating to a magnesium surface said process comprising the steps of cleansing said surface; contacting the surface with an aqueous solution having an elevated temperature and 'consistingessentially of 50-95 parts by weight of sodium pyrophosphate, 1-5 parts by weight of sodium fluoride, and 4-40 parts by weight of a metallic salt selected from the group consisting of cadmium sulfate, copper sulfate, chromium sulfate, nickel sulfate, cadmium cyanide, chromic nitrate, and aluminum nitrate; rinsing the surface; drying the said surface in the air; immersing the said surface in a 540% by Weight aqueous solution having an elevated temperature of one from the group consisting of iron phosphate, ammonium vanadite, and sodium racemes dichromate; and rinsing the surface so that the coated surface exhibits a high degree of uniformity and stability.

2. A process for applying a protective coating to a magnesium surface said process comprising the steps of cleansing said surface; contacting the surface with an aqueous solution having an elevated temperature and consisting essentially of 50-95 parts by weight of sodium pyrophosphate, 1-5 parts by weight of sodium fluoride, and 440 parts by weight of cadmium cyanide whereby the cadmium is plated upon the magnesium surface; rinsing the cadmium coated magnesium surface; drying the i said surface in the air; immersing the said surface in a 40% by weight aqueous solution having an elevated temperature of ammonium vanadite; and rinsing the said surface so that the cadmium coated magnesium surface exhibits a high degree of uniformity and stability.

3. A process for applying a protective coating to a magnesium surface said procms comprising the steps of cleansing said surface; contacting the surface with an aqueous solution having an elevated temperature and consisting essentially of 50-95 parts by weight of sodium pyrophosphate, 1-5 parts by weight of sodium fluoride, and 4-40 parts by weight of nickel sulfate whereby the nickel is plated upon the magnesium surface; rinsing the nickel plated magnesium surface; drying the said sur- References Cited in the file of this patent UNITED STATES PATENTS 1,765,341 Lowe June 17, 1930 2,030,601 McDonald Feb. 11, 1936 2,528,787 Roland Nov. 7, 1950 2,564,864 Thompson Aug. 21, 1951 2,665,231 Amundsen et a1 Jan. 5, 1954 2,758,949 Ley et al Aug. 14, 1956 2,801,197 Bengs July 30, 1957 2,859,144 Jeremias Nov. 4, 1958 2,864,730 Kinder et al Dec. 16, 1958 FOREIGN PATENTS 810,224 Germany Aug. 6, 1951 518,474 Canada Nov. 15, 1955 686,781 Great Britain Jan. 28, 1953 726,935 Great Britain Mar. 23, 1955 

1. A PROCESS FOR APPLYING A PROTECTIVE COATING TO A MAGNESUIM SURFACE SAID PROCESS COMPRISING THE STEPS OF CLEANSING SAID SURFACE, CONTACTING THE SURFACE WITH AN AQUEOUS SOLUTION HAVING AN ELAVATED TEMPERATURE AND CONSISTING ESSENTIALLY OF 50-95 PARTS BY WEIGHT OF SODUIM PYROPHOSPHATE, 1-5 PARTS BY WEIGHT OF SODIUM FLUORIDE, AND 4-40 PARTS BY WEIGHT OF A METALLIC SALT SELECTED FROM THE GROUP CONSISTING OF CADMUIM SULFATE, COPPER SULFATE, CHRONIUM SULFATE, NICKEL SULFATE, CADMIUM CYANIDE, CHROMIC NITRATE, AND ALUMINUM NITRATE, RINSING THE SURFACE, DRYING THE SAID SURFACE IN THE AIR, IMMERSING THE SAID SURFACE IN A 5-40% BY WEIGHT AQUEOUS SOLUTION HAVING AN ELAVATED TEMPERATURE OF ONE FROM THE GROUP CONSISTING OF IRON PHOSPHATE, AMMONIUM VANADITE, AND SODIUM DICHROMATE, AND RINSING THE SURFACE SO THAT THE COATED SURFACE EXHIBITS A HIGH DEGREE OF UNIFORMITY AND STABILITY. 